This invention relates to ion exchange resins, and in particular to mixed-bed ion exchange resins.
Mixed-bed ion exchange resins are well-known to be simple mixtures of the beads of an anion exchange resin with those of a cation exchange resin. When water of another liquid containing dissolved salts flows through a mixed-bed resin, the cation exchange beads exchange hydrogen ions for dissolved cations in the liquid, and the anion exchange beads typically exchange hydroxide ions for dissolved anions. This process normally continues until the available hydrogen or hydroxide ions of the resins have all been exchanged, whereupon these ions must be replaced via the process known as regeneration.
Cation exchange resins are normally regenerated with aqueous solutions of acids, while anion exchange resins are normally regenerated with aqueous solutions of bases. Exposure of the cation resin to the cations of the anion resin regenerant, or of the anion resin to the anions of the cation resin regenerant, would effectively prevent or reverse regeneration, so the resins are typically segregated prior to regeneration. The cation and anion resins are selected with sufficiently different backwash flotation rates, resulting from their densities and bead diameters, that classification of the resin bed by passing water upward through it causes vertical segregation of the two resins within the ion exchange column. Regenerants are then caused to flow within their respective separated resins.
It is well known to increase the thickness of this interface between the two resins by including a third, inert material in the mixed-bed resin composition, which during the ion exchange treatment process is intimately mixed with the ion exchange resins, but which, because of its backwash flotation rate intermediate between those of the cation resin and the anion resin, settles between these two resins during classification prior to regeneration. As disclosed in U.S. Pat. No. 2,666,741, this separation of the cation resin from the anion resin before regeneration allows placement of liquid inlets and outlets between them, reduces the chance of the regenerant for one resin working its way into the other resin, and allows for a reduction in bed size caused by attrition of the resins. In addition, such generation processes are not currently popular.
The spacer materials, which have been useful, typically appear neutral with respect to ion exchange properties. That is, the spacer materials appear ionically neutral, having neither reactive acidic nor basic functional ion exchange sites, and have a backwash flotation rate intermediate between those of the anion and cation resins. For practical considerations, it is helpful if the density of the spacer material approaches a value intermediate between the densities of the two ion exchange resins, so that its particle size and shape may be similar to that of the resins. The spacer material should be at least as physically stable as the ion exchange resins in order that shrinkage of the separation zone does not require addition of spacer material before replacement of the resins. The spacer material must also be resistant to attack by the influent stream and by the regenerants, which are usually solutions of strong acids and strong bases.
Materials which have been used by others as an inert spacer between the anion resins and cation resins during regeneration include beads or particles of polystyrene, polyvinyl chloride, polyethylene and hollow glass spheres. Unfortunately, said spacer material has a tendency for them to aggregate with particles of the anion exchange resin, thus causing the backwash flotation rate of both the resin and the spacer material to be changed. When this occurs, backwashing fails to sharply classify the materials in the ion exchange bed, and the subsequent regeneration is less efficient and less complete. Another problem is the coglomeration or agglomeration of beads caused by trapped gases between beads during backwash. Such a problem causes beads to "float", thus failing to provide a sharp classification of materials in the ion exchange bed.
Mixed-bed ion exchange resin compositions are disclosed in U.S. Pat. No. 4,151,332. Such compositions comprise particles containing in polymerized form, relative large amounts of a hydroxyalkyl methacrylate monomer. Unfortunately, such particles are prepared using polymerization techniques which can yield wide particle size distributions which lower their effectiveness after several regeneration processes. In addition, the relatively large amount of hydrophilic hydroxyalkyl methacrylate monomer which is employed can provide a tendency for the particle to be not water wettable and thus can cause cross contamination of anionic and cationic beads.
In view of the deficiencies of the prior art, it would be highly desirable to provide a particulate spacer material for separation of mixed bead ion resin interfaces, which particles have excellent settling properties when subjected to backwashing, no titratable ionic moieties, and good physical properties.